[clang][Interp] Fix discarding non-primitive function call return values

[tbaeder]

Summary

As briefly discussed in https://reviews.llvm.org/D136013

This creates a temporary local variable and puts the RVO pointer on the stack for the function to write the result into.

[shafik]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

1158

Could you alternatively use Func->hasRVO()?

[tbaeder]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

1158

I don't have to, but now that you mentioned it, that's much better since we won' t allocate the local for void return types... thanks!

[tbaeder]

Ping

[tbaeder]

Ping

[tbaeder]

Ping

[aaron.ballman]

LGTM!

clang/lib/AST/Interp/ByteCodeExprGen.cpp

if (const auto *FuncDecl = dyn_cast_or_null<FunctionDecl>(Callee)) {
     // If the function is being compiled right now, this is a recursive call.
     // In that case, the function can't be valid yet, even though it will be
     // later.
     // If the function is already fully compiled but not constexpr, it was
     // found to be faulty earlier on, so bail out.
     if (Func->isFullyCompiled() && !Func->isConstexpr())
       return false;
 
+    QualType ReturnType = E->getCallReturnType(Ctx.getASTContext());
+    Optional<PrimType> T = classify(ReturnType);
+
+    if (DiscardResult && !T) {
+      // If we need to discard the return value but the function returns its
+      // value via an RVO pointer, we need to create one such pointer just
+      // for this call.
+      if (Optional<unsigned> LocalIndex = allocateLocal(E)) {
+        if (!this->emitGetPtrLocal(*LocalIndex, E))
+          return false;
+      }
+    }
+
     // Put arguments on the stack.
     for (const auto *Arg : E->arguments()) {
       if (!this->visit(Arg))
         return false;
     }
 
     // In any case call the function. The return value will end up on the stack and
     // if the function has RVO, we already have the pointer on the stack to write
     // the result into.
     if (!this->emitCall(Func, E))
       return false;
 
-    QualType ReturnType = E->getCallReturnType(Ctx.getASTContext());
-    if (DiscardResult && !ReturnType->isVoidType()) {
-      Optional<PrimType> T = classify(ReturnType);
-      if (T)
-        return this->emitPop(*T, E);
-      // TODO: This is a RVO function and we need to ignore the return value.
-    }
+    if (DiscardResult && !ReturnType->isVoidType() && T)
 
     return true;
   } else {
     assert(false && "We don't support non-FunctionDecl callees right now.");
   }
 
   return false;
 }

clang/test/AST/Interp/records.cpp

struct S {
   int a = 0;
   constexpr int get5() const { return 5; }
   constexpr void fo() const {
     this; // expected-warning {{expression result unused}} \
           // ref-warning {{expression result unused}}
     this->a; // expected-warning {{expression result unused}} \
              // ref-warning {{expression result unused}}
     get5();
+#if __cplusplus >= 201703L
+    // FIXME: Enable once we support MaterializeConstantExpr properly.
+    getInts();
+#endif
   }
 
   constexpr int m() const {
     fo();
     return 1;
   }
 };
 constexpr S s;